Thyrotropin-releasing hormone (TRH) has been shown to regulate prolactin release and synthesis independently by two separate hormone-receptor interactions, although it has not yet been shown whether these interactions are with two physically distinct sites or one site. To distinguish between these possibilities, analogs of TRH which may inhibit one process and not the other will be tested. Bromocriptine will be tested to see if it regulates prolactin synthesis as well as release, and if analogs have different effects on these two properties. If this is the case, antagonists are available to see if one process and not the other is affected. This will provide functional evidence for one or two receptors. Binding of (3H)dihydroergocriptine to pituitary membranes can be measured directly, and the potencies of agonists and antagonists will be measured and compared to the biological activities. These studies will use GH-cells, a rat tumor cell line, and primary dispersed cultures of rat and pig pituitary glands for studies of biological activities, and rat and pig pituitary gland membranes to measure binding.